Re: [ATM] Betr.: 48" Dob at OMI

[Andy Saulietis <iss@pvtnetworks.net>]

>Lets compare apples to apples.  If you are viewing the Lagoon Nebula at 50x
>in the 8" and at 50x in the 48", are you saying that the stars would be
>brighter but the nebulosity would remain just as dim as in the 8" on both
>scopes?

Per unit area in the eyepiece field of view, yes. Not total brightness which
includes all the light from the object.

You have to compare apples to apples with telescopes concerning 
apparent brightness..
the minimum mag with an 8" scope is about 32 x, with anything less 
all the light from the
scope does not get thru to the eye. A 48" would thus have to have a minimum
magnification of 192x. Remember also that the minimum mag  results in the
maximum possible unit area brightness.


>How would the light reflecting off a speck of dust in the nebula (a point
>source) know it's not coming from a star, so it would know not to get
>brighter in the same fashion that light from a star would?  Seems to me that
>a nebula is just Umpteen-Gazillions of point sources sitting around
>reflecting light.

The apparent total brightness  of a star seen thru a telescope looks
brighter than the view with the unaided eye because both appear to be
point sources to the retina and the eye perceives just the total brightness
with or without the telescope.

An extended object is made up of overlapping point images of the object,
having an angular size that is the same as the resolving power of the 
telescope.
The star-equivalent brightness of one of these points in an extended object
depends on the brightness of that part of the object, but now the eye 
can see the
separate overlapping images so it does not combine these so they look brighter.

The human eye sees large dim objects much better than small dim ones of
the same unit area brightness, and there are some very complex visibility
models which determine how various types of objects appear thru a telescope,
including the level of dark adaptation and other factors.

If the telescope magnification is increased to be larger than the 4x 
per inch of aperture, the
extended objects will appear larger but be fainter per unit area 
since the light is diluted over
a larger area.

The situation is exactly analoguous to viewing a terrestrial scene 
with binoculars or
a telescope, with an exit pupil of 1/4"..the view is not brighter, 
it's just bigger. If
the transmission of the telescope is factored in, the unit area 
brightness is actually
less that that with the unaided eye.

The only way around this is to use an image intensifier or CCD camera..<g>

Andy Saulietis 
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